how can I make the material on the surface different color than the sprinkles on this cube?
To change the color, you must use the Material Properties tab:

To answer your edit, if you want the cube to have a different color than the sprinkles, you have to use different materials for the Cube and Particle Instance objects, respectively.
CUBE MATERIAL

PARTICLE MATERIAL (On sphere and cone)

EDIT - Here is a file with the Cube, as requested: